Leucogranite

In geology, leucogranites are amongst the youngest intrusions related to anatexis of continental crust anywhere in the world. Leucogranites are commonly found in deformed metapelitic/metagraywacke sequences that have been thrusted over basements during crustal thickening associated with continental collisions. Several examples include the leucogranites of the High Himalayas, the Avalon terrane of Maine, and the Black Hills, South Dakota. Leucogranites are typical examples of S-type melts, their composition are strongly influenced by the protolith composition and melt volume depends on the amount of volatiles available during melting and protolith composition

"Leucogranites are common features of convergent orogens and consist of rich-potassium (K2O5.5 wt%), (Ba400 p.p.m.) and Sr (300 p.p.m.), and exhibit low concentrations of Zr (~ 40 ppm), Th (< 1 ppm) and Y (< 10 ppm). The chondrite-normalized REE spectra show that they have a low abundance of (LaN20, LuN3) and are moderately fractionated (LaN/LuN7). An Eu anomaly is absent or weakly negative. The higher 87Sr/86Sr ratio at 550 Ma (0.7345) compared with the migmatite (0.7164) precludes a direct genetic relationship between leptynitic gneisses and leucogranites at Manali. The chemical and mineralogical compositions of the leuocogranites strongly favour a derivation through fluid-absent biotite melting of isotopically distinct but chemically comparable Manali-type gneisses. The undersaturation of Zr, Th and REE, a typical feature of leucogranitic melts generated during granulite facies anatexis of psammo-pelitic lithologies. (Braun et al., 1996).

The source of Leucogranites in the Himalayas are generally ascribed to the aluminous schists and gneisses, partly because of structural relation and partly because of isotopic systematics for Rb, Sr, Nd, O. The monazite and zircon upper intercept ages around 500 Ma of the Zanskar leucogranite clearly indicate that they must be partly derived from the melting of the Cambro-Ordovician Granites. As these old granites are relatively poor in muscovite, the leucogranite must also have had a more pelitic source.

The origin of Himalayan leucogranite was believed to be related to water-saturated melting. Fluid advection from the footwall of the Main Central Thrust (Lesser Himalaya) into the relatively hot hanging wall (HHCS) was invoked as the driving mechanism for the production of water-saturated melt in the upper structural levels of the HHCS (Le Fort et al. 1987). There is however little evidence to support the notion of pervasive aqueous fluids during metamorphism of the mid-lower crust and recent trace element studies argue against fluid present melting in the formation of crustal melts. Also in Zanskar, metamorphic conditions and textural evidences observed in the migmatitic zone rather indicate that melts were derived from the incongruent melting of muscovite.

Lately, Himalayan leucogranitic magmas are thus increasingly thought to be initially water-undersaturated, indicating either that a fluid phase with aH2O<< 1 was present during melting or that the melting reactions were fluid-absent. In the absence of free water, melting depends on the availability in the source region of hydrous minerals like muscovite or biotite which may release their water during anatexis. The amounts of water released by the breakdown of these minerals is usually not sufficient to saturate the magma and is dissolved in the melt without formation of a vapour phase: dehydration melting or vapor-absent melting. Aluminous schists and gneisses are generally considered to be the likely source for peraluminous granites.

The breakdown of muscovite occurs through the reaction: 22 Muscovite + 7 plagioclase + 8 quartz = 5 K-feldspar + 5 Al2SiO5 + 2 biotite + 25 melt, which is also known as the second sillimanite isograd. This reaction can however only produce 10-15 vol. % melt between 750° and 850°C and at 10 kbar. Up to 50 vol. % melt can however be produced at higher temperatures (850°-900°C) with the breakdown of biotite through the reaction: biotite + Al2SiO5 + plagioclase + quartz = Garnet + K-feldspar + melt. The amount of partial melting required before melts begin to segregate and forms plutons is called critical melt percentage. The value of this critical melt percentage is generally believed to be of the order of 25%." (Braun et al. 1996).

References

• Braun I., G.R. Ravindra Kumar, and M. Raithi. (1996) "Dehydration—Melting Phenomena in Leptynitic Gneisses and the Generation of Leucogranites: a Case Study from the Kerala Khondalite Belt, Southern India." Journal of Petrology, Vol. 37, No. 6, pp. 1285-1305.[1]